Inertia compensation for computer-controlled slabbing mills and the like



3,546,907 -CONTROLLED 2 Sheets-Sheet 1.

mmkDn oo LF 1 A. W. SMITH, JR INERTIA COMPENSATION FOR COMPUTER SLABBING MILLS AND THE LIKE 52:28am L H NEE momzmw S wo 5o L 37 momzmm .rzwmmDo 1 4 on momzwm V E @0266 F58 F553 mo 5o E3 Dec. 15; 1970 Filed Oct. 14, 1968 z ooawmow United States Patent ABSTRACT OF THE DISCLOSURE A control system provides for eliminating error in workpiece reduction power determined from actual power readings. Such error is otherwise caused by the power required to accelerate a slabbing mill. The control system employs an on-line computer to operate the slabbing mill as a function of the workpiece reduction power applied to the mill rolls.

CROSS-REFERENCES TO RELATED APPLICATIONS The present invention is related to the invention covered by a copending patent application, Ser. No. 755,633,

filed by A. Smith on Aug. 27, 1968, entitled Improved Method and Computer Control System For Operating A Slabbing Mill and assigned to the assignee of the present application.

BACKGROUND OF THE INVENTION While not necessarily limited thereto, the present invention is particularly adapted for use in the control of reversing slabbing or plate mills. Such mills are conventionally divided into two types. In one type, called a universal mill, ingot thickness is reduced during flat rolling passes by horizontal rolls of a main reversing stand, while ingot width is at least partially reduced during flat rolling passes by separate vertical edger stand rolls located on one side of the main stand. In this type of mill, the ingot is typically placed on edge for scale breaking and partial Width reduction during one or more rolling passes through the main rolls prior to the described flat rolling passes. Under some circumstances, different sequential combinations of flat and edge passes may be employed.

Another type of slabbing mill is the highlift type in which a single reversing stand is provided with horizontal rolls used to produce all of the required workpiece thickness and width reduction. Typically, an ingot might be flat rolled until it is reduced to a first thickness such as 16 inches, and it is then turned on edge and directed through a 16 inch groove in the work rolls for width reduction. Thereafter, the workpiece is fiat rolled to a second thickness such as 10 inches and then turned on edge for a width reduction in a 10 inch groove in the work rolls, and so on.

In the operation of slabbing mills, the ingot enters the mill from one side at a relatively low speed and, thereafter, the mill is accelerated. After the workpiece passes through the mill, its direction of movement is reversed, as are the rolls of the mill; and the workpiece enters the mill from the opposite side, again at a relatively low speed followed by acceleration of the mill. For productivity reasons, it is desirable to process an entry ingot to the specified slab geometry in as little time as possible and with as few back and forth passes through the rolls as possible. This, in turn, depends upon the amount of reduction taken by the rolls during each pass.

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As will be understood, the amount of reduction taken during each pass is dependent upon the hardness of the entering ingot; and this is a function of its temperature, metallurgical composition and possibly other factors. Conventionally, the mill operator determines the ingot size and the required product size and estimates the entry ingot hardness of some characteristic or characteristics which reflect the hardness. On the basis of these factors and on the basis of general experience, the operator chooses the number and types of passes to be made and the amount of reduction to be made on each pass. In the alternative, the slabbing mill may be equipped with a card programmed control, in which case the operator initiates the operation of the control which determines by fixed rules the number and kind of passes to be made.

In either case, mill productivity and mill operating efliciency are limited by the accuracy of the initial estimate of ingot hardness. If the temperature of the ingot or its metallurgical composition is different from specifications, the actual hardness is different from the estimated hardness and a different total work loading is imposed on the mill. This may result in an excess number of passes for a given specified geometry if the ingot is softer than estimated, or too few passes and resulting excessive loading on the mill and its drive motors if the material is harder than estimated.

The actual hardness of the workpiece in a slabbing mill is reflected in the horsepower required to drive the mill rolls as the workpiece passes therethrough. In the previously indicated copending Smith application, computer controlled slabbing mill apparatus is described wherein the workpiece pass and drafting schedule is determined, the average actual horsepower of the mill drive is determined and converted into actual average torque during each pass, and the actual torque is compared to the predicted or estimated torque. If the two are not essentially equal, corrective action is taken by changing the mill schedule to reflect the experienced workpiece hardness in determining within mill constraints the amount of reduction to be taken during each subsequent pass and thereby adaptively maximizing the mill productivity.

While the system described in the aforesaid application is satisfactory for its intended purposes, it does not take into account the error introduced into the actual horsepower reading by virtue of the power required to overcome the inertia and accelerate or decelerate the motor armature, couplings, rolls and the like as the workpiece enters the mill from one side or the other. Thus, while the production of the mill can be greatly improved with the apparatus of the aforesaid copending application, it can be further enhanced if some means were derived to compensate for the power required in the acceleration or deceleration process.

SUMMARY OF THE INVENTION As an overall object, the present invention seeks to provide a system for controlling slabbing mills or the like, wherein the mill draft is adaptively scheduled as a function of the power delivered to the mill rolls and wherein errors introduced into the actual horsepower readings due to acceleration effects are eliminated.

More specifically, an object of the invention is to provide a mill control system of the type described wherein actual horsepower delivered to the mill rolls is derived from a consideration of motor drive armature current, motor drive amature voltage, and the speeds of the rolls at the beginning and end of the acceleration period, re-

0 spectively.

In accordance with the invention, a rolling mill con trol system is provided wherein drive motor armature current and armature voltage are sampled periodically as the workpiece travels through the mill, the sampled currents and the sampled voltages are respectively added, and the sums are multiplied together. There is then subtracted from the resulting product a quantity equal to the product of a predetermined constant times the difference between the squares of the speeds of the rolls at the begin ning and end of the acceleration period, respectively. The latter product is proportional to the energy required to accelerate the rolls from the lower speed to the higher speed. By dividing the difference between the aforesaid products by the number of sampling periods, the actual horsepower required to reduce the workpiece is derived. This is used in a computer to compare average actual torque with computed or desired average torque; and if any appreciable difference exists, an appropriate change in the rolling schedule is made, usually to add passes for the harder ingots and reduce the number of passes for the softer ingots.

The above and other objects and features of the invention will become apparent from the following detailed description, taken in connection with the accompanying drawings which form a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of one embodiment of the invention as applied to a universal reversing slabbing mill;

FIG. 2 is a plot of time versus speed and current under accelerating conditions and steady-state operating conditions for the mill of FIG. 1; and

FIG. 3 illustrates the manner in which the present invention compensates for horsepower errors due to acceleration in controlling the mill screwdowns.

and compares the average actual torque with predicted torque. The latter is derived from a consideration of various factors such as the amount of draft, the composition of the workpiece, the workpiece temperature, etc. If the two are not the same, the mill schedule is updated and the screwdowns are set via leads 54 and 56 for the next pass in accordance with the updated schedule. The computer 46 computes the average torque or horsepower necessary to reduce the workpiece by a given amount under steady-state speed conditions. However, as was mentioned above, the workpiece initially enters the mill at a relatively low speed and is thereafter accelerated to a higher speed.

This is shown, for example, in FIG. 2 where time is plotted against speed and current. After the workpiece enters the rolls at time t and at a rotational speed lRPM(1), it is accelerated until time t where the steadystate speed is RPM(2). At the same time, the current applied to the drive motor 14, for example, increases abruptly from time t to time t reflecting the increased torque necessary to accelerate the motor armature, couplings, rolls and the workpiece itself. This increased torque or power required to accelerate the mill, if noteliminated, con stitutes an error which is introduced into the computations made by the computer 46 in determining the proper screwdown settings for screwdown mechanisms 26 and 2.8.

In accordance with the present invention, average horsepower is determined in the computer 46 by sampling periodically the voltage and current applied to the drive motor or motors, the rotational speed of the motors at the beginning of the acceleration period, and the rotational speed of the motor or motors at the end of the acceleration period in accordance with the following equation:

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference now to the drawings, and particularly to FIG. 1, a universal reversing slabbing mill is shown comprising a pair of horizontal main rolls 10 and 12 which, in the embodiment of the invention shown, are driven by a single drive motor 14. The mill also includes a pair of edger rolls 16 and 20 which engage the sides of the workpiece 22 passing through the mill to reduce its width. The rolls 16 and 20 are both driven by a single drive motor 24; however it will be understood that dual drive motors may be utilized in an actual installation. The spacin between the main rolls 10 and 12 is controlled by a screwdown mechanism 26. Similarly, screwdown mechanism 28 controls the spacing between the edger rolls 1'6 and 20.

Connected to the drive motor 14 for main rolls 10 and 12 is a voltage sensor 30 and a current sensor 32. Similarly, a voltage sensor 34 and current sensor 36 are connected to the drive motor 24. The outputs of sensors 30, 32, 34 and 36 are applied to gates 38, 40, 42 and 44, respectively. These gates, in turn, are enabled by pulses from a pulse generator 44 connected to a roller 45 which engages or follows the movement of the workpiece 22 such that the number of pulses over a period of time from the pulse generator 44 will be proportional to the linear speed of the workpiece 22. As the gates 38, 40, 42 and 44 are enabled, successive samples of voltage and current are fed to a programmed computer 46 having a preset program 48. Also fed to the computer 46 via leads and 52 are the actual positions of the screwdowns 26 and 28.

As more extensively described in the aforementioned copending Smith application, the computer 46 determines the average actual horsepower delivered by the motors 14 and 24, converts this into actual average torque,

where HP=horsepower,

K1 and K2 are constants,

V(1), V(2), etc.=sampled voltages,

C(1), C(2), etc.=sampled currents,

RPM(1)=speed at the beginnin of the acceleration period,

RP-M(2)=speed at the end of the acceleration period,

and

N=number of sampling periods.

With reference, again, to FIG. 2, the samples will be taken between times t and t which includes that period during which the mill is accelerated and horsepower is required to overcome the inertia of the mill and workpiece. This error, however, is eliminated in the foregoing Equation I by the factor K2[RPM(1) RPM(2) this factor being proportional to the energy expended in accelerating the mill and workpiece. When this is subtracted from the product of the sums of the sampled voltages times the sampled currents and divided by the number of sampling periods N, true horsepower required to reduce the workpiece is derived.

In FIG. 3, a schematic flow diagram of one manner in which Equation 1 may be solved by the computer 46 is shown. Voltage pulses from gate 42, for example, are applied to block 48 via lead 50 where the incremental voltage measurements are added over a total number of sampling periods, N. Similarly, current samples on lead 52 are applied to block 54 which adds the incremental current samples over the sampling periods. The sums produced by blocks 48 and 54 are then multiplied in block 56 to produce the'quantity:

[VE(1)+ +VE(N)] [CE(1)+ +CE(N)] At the same time, the actual speed of the mill as determined by the pulse generator 44 is applied via lead 58 to blocks 60 and 62. When the workpiece enters the bite between the rolls as sensed by a strain gage, for example, a signal on lead 64 will store in block 60 the speed at the beginning of the acceleration period, RPME(l). Thereafter, the mill will accelerate, and after the rate of speed no longer increases as detected by a signal from an acceleration detector on lead 64, the steady-state speed of the motor RPME( 2) will be stored in block 62. The representations proportional to RPME(l) and RPME(2) are then squared in blocks 66 and 68; and the outputs of these blocks are substracted in circuit 70 to produce the quantity:

By multiplying the foregoing quantity in multiplier block 72 by the quantity KEZ, the output of the multiplier block 72 will be a representation proportional to:

KE2[RPME(l) RPME(2) This quantity is then subtracted in subtracting block 74 from the output of multiplier block 56 to produce the quantity:

[VE(1)+ +VE(N)] [CE(1)+ This quantity is then divided by the factor N and multiplied by the factor KEl in block 76 to produce the quantity shown in Equation I given above. This quantity is then converted into average actual torque, compared with computed or desired torque in block 78, and any difference is converted into a corrective representation which is used in updating the mill schedule thereby to define new screwdown positioning effected via lead 54 for subsequent workpiece passes.

In a similar manner, voltage pulses from gate 38 are applied to block 80 via lead 82 where incremental volt age measurements across the armature of motor 14 are added over a total number of sampling periods N. Current samples on lead 84 from motor 14 are applied to block 86 which adds the incremental current samples taken over the sampling period. The sums produced by blocks 80 and 86 are then multiplied in block 88 to produce the quantity:

Again, the actual speed of the mill as determined by the pulse generator 44 is applied via lead 58 to computer blocks 90' and 92. When the workpiece enters the bite between the main rolls and 12 as sensed, for example by a strain gage, a signal on lead 94 will store in block 90 the speed at the beginning of the acceleration period, RPMM(l). Thereafter, the mill will accelerate; and after the rate of speed no longer increases as detected by a signal from an acceleration detector on lead 96, the steady-state speed RPMM(2) of the motor 14 will be stored in block 92. Electrical signals proportional to RPMM(l) and RPMM(2) are then squared in squarmg blocks 98 and 100; and the outputs of the squaring blocks are subtracted in subtracting block 102 to produce the quantity:

By multiplying in multiplier block 104 by the constant KMZ, the output of the multiplier block 104 will be a representation proportional to:

This quantity is then subtracted in subtracting block 106 from the output of multiplier block 88 to produce the quantity:

This quantity is then divided by the factor N and multiplied by the factor KMl in block 108 to produce the quantity shown in Equation I given above for the drive motor 14. As was the case for the signals from motor 24, the output of block 108 is then converted into average actual torque in program block 108, and any difference is converted into a corrective representation and an updated schedule. Resultant draft requirements are effected by control of the screwdown 26 via lead 112.

It can be seen, therefore, that the present invention provides, in a system for determining updated scheduling including rolling mill screwdown settings as a function of the power required to drive the mill, means for compensating for errors in the power readings because of the power required to accelerate the mill and the workpiece itself. Although the invention has been shown in connection with a certain specific embodiment, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention. In this respect it will be apparent that the invention is equally applicable to highlift mills wherein only one set of rolls and screwdown are employed, and that separate drive motors can be used for the rolls rather than the single drive motors shown herein. In this latter case, it will be necessary to add voltage and current samples from both motors for each set of rolls in Equation I given above.

What is claimed is:

1. In a control system for a rolling mill wherein a workpiece is accelerated through the mill and a workpiece pass and drafting schedule including screwdown position determined by a comparison of a quantity which varies as a function of the predicted power applied to the mill with a quantity which varies as a function of actual power applied to the mill, the combination of means for determining mill speed, means for periodically sampling current supplied to a drive motor for said mill, means for periodically sampling the voltage applied to said drive motor, means for adding representations of said current samples, means for adding representations of said voltage samples, means for multiplying the sum representations of said voltage and current samples, means for subtracting from the product representation resulting from said multiplying means a representation of the product of a constant times the difference between the squares of the mill speeds at the beginning and end of the acceleration period of the workpiece through said mill, means for dividing the difference between the aforesaid products by the number of sampling periods to derive a quantity which varies as a function of actual power supplied to the mill, means for comparing a representation of the quotient of said dividing means operation with a representation of predicted power, and means for updating the mill schedule and correspondingly adjusting the screwdown as a function of the result of operation of said comparing means.

2. The control system of claim 1 including means for multiplying by a constant the quotient obtained from said dividing means, said comparing means operating with reference to a representation of the product of the latter multiplying means.

3. The control system of claim 1 wherein said rolling mill comprises a reversing mill having main rolls and edger rolls, drive motors for both of said edger rolls and main rolls, and wherein said sampling means periodically samples the currents and voltages supplied to both of said drive motors.

4. The control system of claim 1 wherein said adding and subtracting and dividing and multiplying and comparing and updating and screwdown adjusting means include a programmed computer.

5. In a control system for a rolling mill wherein a workpiece is accelerated through the mill and a workpiece pass and drafting schedule including screwdown position is determined by reference to the predicted power supplied to the mill and actual power supplied to the mill; the improvement of means for eliminating error in determining workpiece reduction power supplied to the mill otherwise due to acceleration efiects in accordance with the equation:

where RPM (1) is the speed of the mill at the beginning of the acceleration period, RPM(2) is the speed of the mill at the end of the acceleration period and K is a constant; said error eliminating means comprising means for pro ducing a first representation proportional to the speed of the mill RPM(1) at the beginning of the acceleration period, means for producing a second representation pro- 'portional to the speed of the mill RPM(2) at the end of the acceleration period, means for producing respective representations proportional to the squares of said speed representations, means for producing a representation proportional to the constant K times the difference of the squared speed representations, means for subtracting the latter representation from a representation which varies as a function of a constant times the measured power supplied to said mill, and means for sensing the motor drive current and voltage to determine the measured mill power.

6. A control system for a rolling mill having workpiece reduction rolls operated by drive motor means, said system comprising means for sensing drive motor current and drive motor voltage during a workpiece pass, means for sensing mill drive speed at least during mill acceleration from base speed to run speed, means responsive to said current and voltage sensing means for determining a representation of the actual power consumed in reducing the workpiece in accordance with a scheduled draft for the pass, means responsive to said sensing means for determining a representation of the power consumed in accelerating the mill, means responsive to the latter two power determining means for determining a representation of the power required for workpiece reduction, means for comparing the workpiece reduction power representation with a predetermined representation of predicted workpiece reduction power, and means for operating the mill in accordance with the result of the latter comparison.

References Cited UNITED STATES PATENTS MILTON S. MEHR, Primary Examiner U.S. Cl. X.R. 72-17, 19 

